Infrared coherent optical sensor

ABSTRACT

A dual-beam amplitude-modulated laser transmitter/receiver suitable for laser-radar applications is scalable to high powers because there is no active modulator element that the laser beam passes through. The transceiver comprises a laser source with two separate independent laser optical cavities. Each laser cavity is similar, but each operates at its own frequency. Signals from the two cavities are superimposed at a combining beam splitter to form two transmitter output beams with each combined beam intensity modulated at the laser difference frequency. The output consists of two beams separated in elevation and with equal beam powers from each laser cavity the intensity modulation is 100%. Each beam has its own homodyne detector and separate local oscillator. Thus, each beam path is considered to be a distinct homodyne transceiver. If the frequency of one of the laser sources is changed in time, an AM/FM/CW output suitable for absolute range measurements and fine doppler is generated. The laser transmitter does not require internal or external modulators such as acousto-optic or electro-optic cells. Thus, it is not limited by the modulator characteristics.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

The present invention relates generally to an infrared coherent opticalsensor and more particularly to a laser transceiver with modulationsuitable for laser-radar applications, including use as a multifunctionsensor.

Pulsed lasers and modulated lasers using various internal and externalmodulators are known in the art for various applications, includinglaser radar.

U.S. patents of interest include U.S. Pat. No. 3,573,463 to Goodwin etal., which discloses a communications system comprising a pair of lasertransceivers. Each transceiver includes two laser beams of differentfrequencies, one of which is modulated according to an informationsignal. The two beams are mixed by means of a beam splitter which may bedisposed either inside or outside the laser cavity. U.S. Pat. No.3,409,369 to Bickel discloses a laser radar doppler shift system usingtwo transmitted frequencies. Since the two frequencies are slightlydifferent, the beat frequencies go in and out of phase, causing anamplitude modulation whose frequency is proportional to the differencein the two beat signals and the velocity of the target. U.S. Pat. No.3,258,597 to Forrester shows a laser communication system havingseparate spectral line sources 10 and 11. Fried et al in U.S. Pat. No.3,302,027 describe an interferometric arrangement for modulating light.U.S. Pat. No. 4,195,221 to Moran is directed to an optical heterodynescanning system in which two coherent light signals are mixed to producea different signal and Forrester et al in U.S. Pat. No. 4,391,515 showan optical transceiver with shared common optics.

SUMMARY OF THE INVENTION

An object of the invention is to provide a simple, efficient,amplitude-modulated laser transceiver suitable for laser-radarapplications, that is scalable to high powers.

The invention is directed to a dual-beam amplitude-modulated lasertransmitter/receiver suitable for laser-radar applications that isscalable to high powers because there is no active modulator elementthat the laser beam passes through. The transceiver comprises a lasersource with two separate independent laser optical cavities. Each lasercavity is similar, but each operates at its own frequency. Signals fromthe two cavities are superimposed at a combining beam splitter to formtwo transmitter output beams with each combined beam intensity modulatedat the laser difference frequency. The output consists of two beamsseparated in elevation and with equal beam powers from each laser cavitythe intensity modulation is 100%. Each beam has its own homodynedetector and separate local oscillator. Thus, there considered to be adistinct homodyne transceiver associated with each beam path. If thefrequency of one of the laser sources is changed in time, an AM/FM/CWoutput suitable for absolute range measurements and fine doppler isgenerated.

A feature is that the laser transmitter does not require internal orexternal modulators such as acousto-optic or electro-optic cells. Thus,it is not limited by the modulator characteristics.

Advantages of the invention are that it is much more efficient than theprior art devices because it does not require a modulator with itsdriver, it is easy to change modulation frequency or to have it varywith time because this is done electronically at low power levels, andit is scalable to high powers because there is no active modulatorelement that the laser beam passes through.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a symbolic diagram of a lossless modulation transmitter;

FIG. 2 is a symbolic diagram showing lossless modulation basic receiverprocessing:

FIG. 3 is a schematic and block diagram of a dual-beam MultifunctionInfrared Coherent Optical Sensor (MICOS);

FIG. 3A is a symbolic diagram of an alternative showing dynamic beamcombining;

FIG. 4 is a block diagram showing injection locking;

FIG. 5 is a diagram with graphs showing FC sensor scanning by mode;

FIG. 6 is a block diagram of a FC sensor system; and

FIG. 7 is a functional block diagram of the FC sensor.

DETAILED DESCRIPTION Outline

1.0 Lossless Envelope Modulation

1.1 Lossless Envelope Modulation Transmitter

1.11 Optical Characteristics

1.12 Electrical Characteristics

1.2 Lossless Envelope Modulation Basic Receiver Processing

1.21 Optical Characteristics

1.22 Electrical Characteristics

1.23 System Application

2.0 MICOS Introduction

2.1 Dual Beam MICOS

3-D Mode

MTI Mode

WD Mode

2.2 Injection-Locking Frequency Control

2.3 Discussion and Conclusion

3.0 MICOS System Description

FC Sensor

3.1 FC SENSOR OPERATION

1.0 LOSSLESS ENVELOPE MODULATION

The system comprises a transmitter and a receiver, which as shown laterare combined as a transceiver.

1.1 LOSSLESS ENVELOPE MODULATION TRANSMITTER

In lossless envelope modulation, two similar laser source beamsdiffering in frequency are optically combined to form two output beams.The envelope and Power of each output beam is modulated at the frequencydifference between the two source beams. Since all the laser powerappears as 100% modulated output, this method achieves the maximumpossible efficiency.

1.11 Optical Characteristics

Lossless envelope modulation is illustrated in FIG. 1. This shows twosimilar laser beams combined at a beam splitter. The essential opticalcharacteristics of this figure are:

(a) The laser beam 112 from the "fixed-frequency" source 110, showndotted, is turned by a mirror 114 to the 50% beam splitter 116.

(b) One-half of the power of this laser beam 112 passes upward throughthe beam splitter 116 and one-half of the power is reflected to theright.

(c) The portion of the beam that passes upward is reflected to the rightby a turning mirror 118.

(d) In similar fashion, the laser beam 122 from the "tunable-frequency"source 120, shown dashed, transmits through and reflects from the beamsplitter 116.

(e) The tunable-frequency dashed beam is thus split and superimposed onthe fixed-frequency dotted beam.

There are two output beams, #1 and #2. Each of these contains one-halfof the power from the individual laser sources.

1.12 Electrical Characteristics

The essential electrical characteristics of this coherent opticalcombining are also illustrated in the figure:

(1) The tunable-frequency and the fixed-frequency laser beams are shownas sine waves.

(2) In this illustration, the dashed-beam sine wave 122 is shown as alower frequency than the dotted-beam sine wave 112.

(3) The output beams #1 and #2 contain both sine waves equally.

(4) Each output beam is a 100% envelope-modulated sine wave.

(5) The frequency of the envelope modulation is the frequency differencebetween the two source beams.

(6) The high-frequency carrier in the modulated output beams is theaverage of the two laser source frequencies.

Thus, in lossless envelope modulation, the laser transmitter generatestwo output beams. Each beam is offset in angle from the other at theoutput aperture to form two scan beams.

1.2 LOSSLESS ENVELOPE MODULATION BASIC RECEIVER PROCESSING

The laser receiver detects and uses the envelope modulation. The laserdifference frequency and resulting envelope modulation areelectronically controlled to satisfy the various sensing functions. Forexample, a fixed-difference frequency near 10 Mhz would be used forhigh-resolution 3-D. A swept or changing frequency difference would beused for the measurement of absolute range.

The receiver has two detection channels. Each channel is angularlyaligned to receive scattered laser energy from its respectivetransmitter beam. In the transmitter, two laser sources differing infrequency were combined spatially to synthesize the envelope-modulatedbeams.

The receiver essentially regenerates electrically the originaltransmitted signal format. For 3-D measurements, the frequency of theenvelope modulation is fixed and the electrical signals are used todetermine the received envelope phase for comparison with thetransmitter phase. For range measurements, the frequency of the envelopemodulation is changing and the received envelope frequency is measuredand compared with that from the transmitter.

1.21 Optical Characteristics

The basic receiver processing elements are illustrated in FIG. 2, whichshows one of the receiving channels. The received scattered signal 201is spatially combined in a beam combiner 210 with a portion of thefixed-frequency laser transmitter source as a local oscillator signal202. The received signal 201 and local oscillator signal 202 thus form abeam that is intensity modulated at the frequency differences betweenthe signal and the local oscillator. This frequency difference which isa consequence of the doppler shift serves as the carrier for the signalenvelope modulation. A detector 212 converts the intensity modulation tocurrent and, as shown in the figure, the signal envelope modulation ison the detector current on line 214.

1.22 Electrical Characteristics

The electric input to the receiver is an envelope-modulated signalsimilar to the output of the transmitter. However, the carrierillustrated in the receiver figure is about 5×10⁷ Hz for operation nearMach-1. The transmitter optical carrier illustrated in FIG. 1 is 3×10¹³Hz for 10 micrometer CO₂ lasers.

The receiver doppler unit 216 tracks the detector signal to utilizenarrowband noise filtering in order to achieve high sensitivity. Thesignal envelope is then detected and measurements of the phase andfrequency on this envelope as shown by blocks 220 and 222 are made andcompared with a reference 224 from the transmitter.

1.23 System Application

Since the receiver can be configured to utilize the frequency of thetransmitted envelope in its fixed processing circuitry, the receiver isinherently adaptable to fixed-frequency and variable-frequency envelopemodulation. Thus, changes in modulation format can be used to enhancethe system utility. For example, scan lines for absolute rangemeasurement can be interleaved with scan lines for 3-D relative rangemeasurement during one frame.

2.0 INTRODUCTION

The system makes use of a multifunction sensor in the form of a CO₂laser radar sensor for detecting, classifying and attacking movingground targets from a low-flying high-performance aircraft.

Advances in the state-of-the-art Hg:Cd:Te photodiodes, production ofhomogeneous optical material and laser frequency control were developedto produce the sensor. Perhaps the most noteworthy of the advances isthe essentially lossless technique for producing amplitude modulation ofthe transmitted laser beams. In addition to a description of thesedevelopments, the report describes design details of the sensor, bymajor subassembly, and shows the results of tests performed to evaluateperformance. A Terrain Clearance capability is provided by a second CO₂laser radar sensor.

Hardware was designed and developed to provide attack and fighteraircraft with the following capabilities:

Terrain clearance control for low level flight.

Autonomous target attack by performing the following functions:

Detection of moving targets.

Automatic target classification.

Guiding ripple fired hyper-velocity missiles.

2.1 DUAL BEAM TRANSCEIVER

The basic dual-beam transceiver is shown schematically in FIG. 3. Theoutput consists of two beams separated in elevation. Although each beamhas its own path through the optical train, the beams share the scanelements and optics. Each beam has its own homodyne detector andseparate laser local oscillator. Thus, there can be considered to be adistinct homodyne transceiver associated with each beam path.

During operation, the beams are scanned together and each beam scans aseparate line. The angular spacing between the beams, and theirresulting line spacing is determined by an element such as the prismunit 320 shown. This line spacing can be: (1) fixed as a compromise thatsatisfies the MTI, 3-D and WD modes (2) changed for each mode, or (3)under dynamic galvo control. This latter possibility is shownschematically in FIG. 3A, using a dynamic beam combining unit 321 inplace of the prism unit 320.

From the prism unit 320, the two output beams pass through an azimuthscanner wheel 380, a relay lens group 382, an elevation scangalvonometer 384, another relay lens group 386, an azimuth pointergalvonometer 388, and a final afocal lens group 390.

For the MTI search mode or the 3-D target identification mode, a givenfield must be scanned in a given time. The signal to noise at thereceiver IF is proportional to the power on the target and inverselyproportional to the bandwidth. When the dual-transceiver design iscompared with a single-transceiver design the power on target for eachchannel is halved but the bandwidth is also halved because of thereduced dwell time on each pixel. Thus, the basic performance for thedual-beam transceiver is the same as for a single-beam transceiver. Thelonger dwell time with the dual design can be used to improve thedoppler or MTI resolution.

The transmitter source consists of a laser design with two separateindependent laser optical cavities 310 and 320. Each laser cavity issimilar but each operates at its own frequency. A small sample of thebeam from each laser cavity is used as the local oscillator beam in thishomodyne configuration. This beam is rotated to orthogonal polarizationby a λ/2 plate, optically shaped by the homodyne lens group to form asource that uniformly illuminates the detector plane after passingthrough the polarizing beam splitter and the detector lens to thedetector.

From laser cavity 310, the beam 312 reflected by a polarizing beamsplitter 332, passes through beam splitter 334, the λ/2 plate 336,reflected by a mirror 338, thence through the homodyne lens group 350,and a detecter lens 352 to a homodyne detector, 354. From laser cavity320, the beam 322 reflected by a polarizing beam splitter 342, passesthrough beam splitters 344, the λ/2 plate 346, reflected by a mirror348, thence through the homodyne lens group 360, and a detecter lens 362to a homodyne detector, 364.

3-D Mode

For 3-D amplitude modulation, the outputs from the two cavities 310 and320 are accurately superimposed at the combining beam splitter 316 toform two beams 315 and 319 for transmitter output. Each combined beam isintensity modulated at the laser difference frequency. With equal beampowers from each laser cavity, the intensity modulation is about 100%.Since the two modulated output beams do not overlap in the targetregion, operation does not depend on the phasing between the two arms ofthe combining 50 beam splitter. Therefore, since only the angularalignment of the interferometer must be held accurately, this techniqueplaces no new demands on the usual optical design.

The two laser cavity frequencies are shown controlled at the desiredoffset frequency by means of an electronic feedback loop. A small sampleof one of the superimposed output beams illuminates the feedback controldetector 372. The detected laser frequency difference is compared withthe frequency of a reference oscillator 370 in the phase detector 374,the error signal is used in unit 376 to lock the frequency of laser #2(310) to laser #1 (320). Laser #1 is controlled by means of a low-dutyintermittent dither loop using a dither control unit 378 in ahill-climbing servo configuration, to maintain its frequency near itsmaximum power output point. Since the modulation rates are so muchdifferent, the same detector 374 can be used for both loops.

The 3-D measurement accuracy depends critically on knowing orstabilizing the modulation frequency, i.e., the laser frequencydifference. The influence of the stability of this difference frequencyon delta-range accuracy can be determined by first considering the laserphase difference Δφ. The allowable laser frequency shift Δf to achieve agiven delta-range measurement accuracy of ΔR when the modulationfrequency is f_(m) is given by

    Δf≦(Δφ/2π)Δt               (1)

This can be written as

    Δf≦f.sub.m ΔR/R                         (2)

where R is the range. A 20 MHz difference frequency (or a modulationwavelength of 15 m) a maximum range of 5 km and an R of 0.3 m givesf≦1.2 kHz. This short-term stability is achievable from a single laser;however, extremely careful designs in the areas of stable cavities,stable discharges, PZT mount design and control-loop electronics arerequired to achieve this frequency tracking control over the full timethat a target is being measured (times of greater than 100 ms at shortranges).

As described later, laser injection locking in conjunction with laserfrequency tracking can produce this required laser tracking stabilityand decrease the design burden in this area.

MTI Mode

During the MTI tracking mode, no intensity modulation is required. Thisoperation corresponds schematically to removal of the 50% beam splitter316 in the combining interferometer. Each laser can now operateindependently at any frequency and the system will perform in this mode.However, the control of the two laser frequencies at the offset desiredfor 3-D is not deleterious to this mode. Thus, the apparent preferredoperation is to have the lasers controlled and tracking for the 3-D modeduring the MTI search. The lasers are now always ready and 3-D can beinitiated any time the combining beam splitter is in place.

WD Mode

During weapon delivery a number of possibilities can be implementeddepending on the guidance scheme. The targets will be tracked using MTIduring this mode. Since only one beam is required to perform MTI duringWD, the other beam can be coded or modulated as required. For example,beam polarization can be used to code positions within a targetingframe. In this case, the scan can always start from the bottom with theuncoded target line scan occurring first followed by the missile line.Thus when a missile gets a signal it merely uses the firstcircular-polarized beam as an initiating signal and then uses theinformation from the following missile line scan to determine itsposition. The order of events could, of course, be changed withdifferent logic at the missile. Since the two beams are offset in anglethere is no confusion which is the target beam and which is the missilebeam. For this example, the laser frequencies can certainly remainlocked at their 3-D separation.

As another example, amplitude modulation can be used to code the beamposition. In this case, a Pockels cell similar to that used forpolarization modulation can also be inserted in the missile beam and thesequence is similar to that described above. Alternatively, the laseroutputs can be superimposed as in 3-D and offset in frequency using thelocking loop to tune the modulation between, for example, 1 MHz and 2MHz. These low frequency differences accommodate the missile detectorresponse. However, as described before, the basic laser stability andthe ability to control frequency limits the utility of this method. Adynamic injection-locking loop would be more difficult to implement inthis range of frequencies.

The WD mode can use a beam that is separate and distinct from the MTItarget-tracking beam. During the initial missile launch period when wideangles must be scanned at high rates, this separate beam approach cangive additional system design flexibility.

2.2 INJECTION-LOCKING FREQUENCY CONTROL

The mixing of the outputs from a two-cavity laser can be used toefficiently generate the laser amplitude modulation required in 3-Dmeasurements. With this method, high power intracavity electro-opticmodulators which still must be proved are not necessary or theinherently less efficient external modulators are not required. Themodulation is instead accomplished by means of passive optics andfrequency control of lasers. The frequency control is accomplished withelectronic techniques which are quite common now.

However, as described previously, the control of the frequency offsetthat is required during a 3-D measurement time is difficult with onlyelectronic, mechanical and electromechanical design. The locking of twoCO₂ laser frequencies together using injection-locking techniques hasbeen studied extensively and is suitable for satisfying the 3-Drequirements. The optical schematic showing injection-locking frequencycontrol is shown in FIG. 4. A small amount of laser power from the powerstabilized laser 410, shown exiting from a low transmissivity backmirror 412, is shifted by the modulation frequency required with a GEacousto-optic cell 414. Then, this frequency shifted power is injectedinto the other laser cavity 420. This injected signal, which is withinthe frequency locking range of the second laser, controls the frequencyof the second laser. The two outputs are now offset in frequency by thefrequency drive in the AO cell 414 and the required difference frequencystability which depends on the drive to the AO cell is now readilyachieved.

The original setting of the two cavities for the injected power to bewithin the injection-locking region is accomplished with the electroniccontrol loop. Thus, initially the two laser frequencies are set veryclose to their separation electronically then the injection processstabilizes them to the degree required. If a new laser modulationfrequency is required, the electronic loop is tuned to select thisoffset. In addition, the AO modulated beam is mechanically rotated to anew angular position to line up with the entrance angle to the laser.This can be accomplished automatically, as the frequency difference istuned by means of a mechanical-tracking mount which also rotates thecrystal to maximize the Bragg effect.

The design characteristics of the injection-locking technique can beevaluated by determining the injected laser power required to make theinjection-locking region greater than the laser frequency associatedwith the electronic control loop.

The frequency locking region for the cavity illustrated in FIG. 4 can bewritten as (see C. J. Buczek, R. J. Freibert and M. L. Skolnick, "LaserInjection Locking", Proc. IEEE, vol 61 pp. 1411-1431 Oct. 1973.)

    f=(Fc/2L)2(P.sub.o /P.sub.osc).sup.1/2 (TT.sub.osc).sup.1/2(3)

Here (c/2L) is the cavity axial-mode spacing. (P_(o) /P_(osc)) is theratio of the injected power to the self oscillating power. T is thetransmission of the injection port and T_(osc) is the overalltransmission of the oscillator. For the waveguide laser design underconsideration, the following are reasonably characteristic values.

    c/2L=150 MHz

    (TT.sub.osc).sup.1/2 =(0.005×0.2).sup.1/2 =0.03.

Thus, the power ratio (P_(o) /P_(osc)) required to achieve a Δf of 20kHz, which can be reasonably obtained from the electronic control, isapproximately 5×10⁻⁶. For a 20 w output laser, this means that 100 μw ofinjected power is required to establish a 20 kHz locking region. Thispower is very small and easily obtained. Another approach is to considerwhat is the size of the injection-locking region for the followingreasonable laser design parameters. The laser power before frequencyshifting in the AO cell is 0.5 w, the low-drive AO cell is 5% efficient,the oscillator power is 20 w, the back mirror transmission is 0.005 andthe output mirror transmission is 0.2. For the same c/2L as usedpreviously, this gives us an injection-locking region Δf=300 kHz. Thisis well in excess of the requirements. Thus, the parameters required toachieve precise injection control are rather easily achieved and thispowerful laser technique can be utilized to greatly simplify the designof the 3-D laser source. The laser injection-locking requirements aresufficiently modest that wide optical flexibility exists in implementingthis technique. In fact the injection power requirements are low enoughthat a small, low-drive power, in-line electro-optic modulator should beconsidered for the frequency shifter. This element does not have to betuned angularly in position as frequency is changed. The EO frequencyshifter would certainly be more suitable than the AO cell for use in theagile lower frequency WD scan discussed previously.

2.3 DISCUSSION AND CONCLUSION

The dual-beam MICOS approach does satisfy the system requirements withcw lasers in a configuration where almost all the laser power isutilized effectively. No higher power lossy AO or EO modulators arerequired. Laser injection-locking, a powerful technique in CO₂ lasers,is used to stabilize the modulation frequency.

The dual-beam MICOS does require additional receiver detectors andapparently more receiver processing. However, there is an inherentflexibility in this geometry which compensates for this. Moreover, theseadditions which increase the effective usage of transmitter power are inthe low weight, low drain Portions of the equipment.

3.0 MICOS SYSTEM DESCRIPTION

In a system designated MICOS-1, major system functions of AutomaticTarget Acquisition/Enhanced Classification Potential/Weapon Delivery(ATA/ECP/WD) grouped as fire control are performed by an FC sensor,andthe Terrain Clearance function is provided by a TC sensor.

Both MICOS-1 Sensors employ coherent detection to obtain maximumsensitivity and in the case of the FC Sensor to provide a Moving TargetIndication (MTI) discriminant. Usually,the FC Sensor is described asoperating in the homodyne form of coherent detection; however, asdescribed in the MICOS report, Vol. II, it also operates in theheterodyne form. The TC Sensor operates only in the heterodyne form ofcoherent detection. In an operational configuration, both sensors areenvisioned installed in a pod that is attached to a high-performanceaircraft and interfaced with the host aircraft avionics to provideautonomous operation. The FC sensor includes an embodiment of theLossless Envelope Modulation using a dual beam laser.

FC Sensor

With the host aircraft flying at an altitude of 60 meters above groundlevel (AGL), at speeds up to 0.9 mach., the FC Sensor will perform theATA, ECP, and WD functions in time sequence. In the ATA mode, anelevation pointing mirror in the sensor is driven by a galvonometer tosinusoidally scan 52.4 milliradians (3°) P-P in object space at afrequency of 300 Hertz. A raster 698 mrad (40°) wide is generated bysweeping the 103 mrad (5.9°)×103 mrad narrow field-of-view (NFOV) bymeans of a mirror external to the sensor. This mirror also is controlledby servos (part of the pod electronics) to provide sightlinestabilization for the Sensor. Actually two laser beams, having a fixedangular separation in each mode, are scanned. One beam is co-alignedwith the optical axis of the sensor; and the other beam is to the right(in azimuth) and above (in elevation) the on-axis beam (viewing thebeams in the far field from the sensor). In the ATA mode, the angularseparation in azimuth (0.58 mrad) and the azimuth sweep rate are set toassure that at least one beam will intercept a 3.4-meter long target ata range of 4.5 km.

Only the half cycle of the scan from the bottom to the top (closestrange to the longest range) is used, as shown in FIG. 5. (Note: For thepurpose of better illustrating the sensor operating modes, the relativesize of the rasters and targets is not correct. The target (tank) in theexpanded ATA raster should subtend more than two scans in azimuth.)Since the pod was not included under the MICOS-1 project, the azimuthcomponent of the ATA raster is swept by the azimuth pointing mirror inthe sensor, thus the dimensions of the raster are 52.4 mrad P-P inelevation by 103 mrad in azimuth. Six hundred cycles, scanned togenerate the 698 mrad wide raster, produce 1200 cycles (and activescans) in object space, whereas 88.5 cycles are scanned to generate the103 mrad wide raster to produce 177 cycles and active scans.

Detection of an MTI cue when either beam, or both beams, intercept amoving target, while scanning the ATA raster, causes the targetcoordinates to be stored in a target file. In a tactical system, thecoordinates would be in inertial space. The position of the galvanometerthat drives the lag angle compensator (corrects for the angle throughwhich the beam is scanned during the round trip time, at the speed oflight, to the target) also is read when the MTI cue is detected. Thisangle, and the scan rate, are used to calculate coarse range to thetarget, which also is stored in the target track file with the targetcoordinates. Up to 19 or 20 target files can be created in theoperational system configuration.

After the ATA raster has been scanned, the sensor sequences to the ECPmode and the line-of-sight (LOS) of the sensor is commanded to thecoordinates of the target in the first target file. Because the targetwas moving when detected, it is not likely to be exactly at thesecoordinates when the ECP mode is initiated, so an Automatic TargetReacquisition (ATR) raster is scanned to relocate the target. Thisraster is biased to start 22 mrad to the left of the target azimuthposition in the target file as shown in FIG. 5. (No offset in elevationis needed because only minor variations in this angle will be evident toa low flying aircraft.) It (raster) is 8 mrad P-P in elevation, scannedat 300 Hz sinusoidally with the same beam spacing and azimuth sweep rateas the ATA raster. When a MTI cue from the target is received, scanningof the ATR raster is terminated, the new target coordinates are storedand the sensor is readied to scan an ECP raster. If an MTI cue has notbeen received by the time the ATR rater is 44 mrad wide, scanning isterminated, the target is dropped from the file and the LOS is commandedas described above to the coordinates of the target in the second targetfile.

After scanning of the ATR raster is terminated upon receipt of an MTIcue, the angular separation between the beams in azimuth is reduced to0.1 mrad; i.e.. essentially contiguous, for scanning the ECP raster. (Toprevent crosstalk between the signals produced by both beams, theoff-axis beam is 0.2 mrad above the on-axis beam in the NFOV and 0.62mrad in the WFOV.) Another pair of galvonometer rotated mirrors(scanners), in the same coordinate system as the pointer mirrors, aredriven to produce the ECP raster. The elevation scanner is driven with a300 Hz triangular waveform to provide a 4.2 mrad P-P raster in objectspace. The AZ pointer and AZ scanner are commanded so that the ECPraster is started two mrad to the left of the target azimuth positiondetermined when the MTI cue was detected in the ATR raster. Thisprecaution is taken to assure that the ECP encloses the left edge of thetarget. The EL pointer is held at the angle read out when the MTI cuewas detected in the ATR raster. Coordinates of the target leading edgeand trailing edge plus relative range to the target; i.e., surfacefeatures, are provided to a target classifier as this raster is scanned.Logic in the sensor terminates the ECP raster when two scans (from thebottom to the top of the raster, as in ATA and ATR) are completedfollowing the last MTI cue. If this condition is not met, the raster isterminated when it is 10.6 mrad wide.

When the ECP raster is terminated, the sensor parameters for scanning anATR raster are restored, the target in the second target file isaddressed and the above sequence is repeated. In a tacticalconfiguration, each target detected during ATA would be classified insequence until up to six high priority targets have been identified.

Following completion of the ECP sequence, the sensor is configured fortracking the selected targets (six maximum); i.e. the Automatic TargetTracking (ATT) mode. This is accomplished by inserting the WFOV lensgroup in the optical path of the Afocal Telescope and bringing the WFOVsegmenting prisms together in front (object side) of the objective.These prisms bisect the objective vertically; i.e., the verticaldimension of the 373.5 mrad (21.4°)×373.5 mrad WFOV is split. The prismangle and orientation cause the WFOV to be reformatted so that thesensor views +57.6 mrad (±3.3°) EL×663.2 mrad (38°) AZ.

Target tracking rasters, generated by the scanners, can be centered ontargets anywhere in the WFOV by applying appropriate commands to thegalvanometer servos for the pointer mirrors. That is; the WFOV Pupil(3.329 cm dia.) can be positioned anywhere on the 12.7 cm objectiveunder control of the pointers, and thereby to a corresponding positionon a prism for directing the line-of-sight (LOS) as desired. Targettracking rasters 12.7 mrad square are scanned sinusoidally in elevationat 960 Hz with contiguous beam separation, while target data areacquired in each direction of scan. They are scanned at a rate of 48 persecond with 16.667 ms of each interval expended to scan the raster (16cycles) and 4.1667 ms used to command the pointers to the next target.Thus six targets can be tracked with position updates eight times persecond.

Target tracking is initiated immediately following insertion of the WFOVlens group and the segmenting prisms. The AZ and EL pointers arecommanded to the expected coordinates of each target in sequence. Thesecoordinates are extrapolated from the apparent velocity in the azimuthaxis, computed from the target position determined in the ATA mode andin the ECP mode. The target position error found by scanning the firstATT raster is used to update the target position before closing thetarget tracking loop. This is done to avoid the potential application ofa large transient to the loop at the start of tracking. The targettracking loop then is closed after the second raster has been scanned.

A precision clock controls the timing of the ATT rasters andsynchronizes the scanners so that the scan or elevation waveform, alwaysstarts at the same position (spatially) at the beginning of each raster.This feature is required for compatibility with a missile terminalguidance concept based upon logic in a missile receiver that determinesposition in the raster from timing of the raster scans detected by thereceiver. A lower power coarse raster for missile capture also is arequirement of the above concept.

3.1 FC SENSOR OPERATION

FIG. 6 is a system block diagram of the FC Sensor. It employs a dualbeam dual frequency transmitter 610, which is a dual beam CO₂ waveguidelaser that is RF pumped. Good thermal stability attenuation of externalacoustic disturbances is obtained by optimum use of invar in theconstruction of the device. One beam of the laser is stabilized on thepeak of its gain curve and the other beam is coherently slaved to itwith an 8 MHz offset.

The angular beam separation mentioned in section 3.0 is provided by thedual beam variable angular offset optics 612 following the dual beamtransmitter (laser). Both beams, with small angular separation, passthrough the T/R (Transmit/ Receive) duplexer 614 and lag anglecompensator 616 to the Scanner/Pointer 620, comprising an elevationscanner 622, an azimuth scanner 624, an elevation pointer 626 and anazimuth pointer 628. The scanners and pointers operate in the sameorthoqonal coordinate system, as already mentioned. The scanner mirrorsare small, low inertia beryllium elements, whereas larger mirrors(brazed beryllium assemblies) are required for the pointers toaccommodate an increased beam diameter provided by the beam expanderbetween the scanner and pointer sections.

Following the Scanner/Pointer 620 there is an Afocal optics group 630comprising the WFOV and NFOV optics 632, the Afocal Telescope 634, FOVsegmentation wedges (prisms) 636 and an azimuth pointing mirror 638. TheAfocal Telescope 634 relays the pupil at the azimuth pointing mirror 628in the Scanner/Pointer 620 to a collimated ray bundle that is coaxialwith the yaw gimbal of the pod head. This collimated ray bundle is theinput to an afocal beam expander on the gimbals of the stabilized sight.The objective of the Afocal Telescope is one element of this beamexpander. A separate four-element lens group can be insertedelectrommechanically between the elements of the beam expander toincrease the field of view by a factor of almost four.

Transmitted power scattered from object space retraces the optical paththrough optics 630, 620 and 616 to the T/R duplexer 614, from which itis reflected to the a quad detecting receiver 640. The signalcorresponding to each beam is imaged on a separate detector pair.Compensation for lag angle is provided by driving the lag anglecompensator 616 with an analog signal representing a flat earthprediction and correcting for any departure from the prediction by meansof an error signal proportional to the position of the image on onedetector pair.

Electronic output signals from the detector 640 are supplied to thedetector electronic circuits 650. Upper and lower sidebands areelectronically heterodyned to fixed IF amplifiers in the upper and lowersideband signal processors 652 and 654. The upper sideband from unit 652is used in a target doppler MTI processor 656 to detect target doppler.The lower sideband from unit 654 is processed in a relative rangeprocessor 658 to provide one of the two inputs needed to develop the 8MHz signal used by a digital phase detector to obtain the relativerange, or ΔR output. The reference signal used by the phase detector isprovided by the servo electronics for the dual beam transmitter.

Operation of the sensor is under control of a microprocessor programmedto sequence the sensor through the three modes described in section 3.0.A secondary microprocessor, under control of the primary microprocessor,performs the arithmetic intensive operations such as coarse rangecomputation and mechanization of the target tracking filters. All of thescan waveforms for raster generation and timing plus the A/D and D/Aconverters are included with the control electronics assembly.

FIG. 7 is a functional block diagram of the FC Sensor, less the AfocalTelescope which has been omitted to keep the figure from becoming too"busy". The sources of power for this sensor is the dual beam laser 610.The cavity structure of the laser is a classical "sandwich" of fivealuminum electrodes retained between two pieces of alumina to form fourbores and to provide electrical isolation. The figure shows four beams701-704 in the four bores. Two of the bores With beams 701 and 702 areoptically coupled via mirrors 705 and 706 to form the cavity of onelaser and the other two bores with beams 703 and 704 are coupled in thesame manner via mirrors 707 and 708 to form the second laser cavity. Allcomponents of the lasers are installed in an invar housing. The lengthof each cavity is controlled by a PZT that displaces the totallyreflecting mirror. One PZT 711 is in a closed loop servo 713 (hillclimbing servo) that keeps the laser operating at the peak of its gaincurve using the signal provided by the pyro electric detector 774 whichsamples the outdoing beam 732 using beam splitter 718. The PZT 712 forthe other cavity is in a closed-loop servo 775 (wavelength offset servo)that keeps the second laser coherently offset by 8 MHz from the firstlaser using the signal provided by the pyro electric detector 772 whichsamples the outgoing beam 732 using beam splitter 717.

The unique essentially lossless modulation feature to produce amplitudemodulation of the transmitted beams is accomplished by means of the 50percent beamsplitter 716 that is common to both beams 702 and 704 of thelaser, beam 702 being directed via a mirror 714 and the other beam 704being directed via mirrors 715 and 717 to opposite sides of the beamsplitter 716. As a result, both beams 722 and 724 out of the beamsplitter are composed of one-half of the output of each laser and bycareful optical alignment they will have a common optical axis. Sincethe components are separated in frequency by 8 MHz they will be inphase, then out of phase twice per cycle at 8 MHz: thereby producingamplitude modulation at 8 MHz.

One of the beams 732 from the 50 percent beam splitter 716 is notdeviated by the lens 730 following the beam separation mirrors 732 and734. However, it can be seen that linear translation of one beamseparation mirror causes the other beam 724 to be deviatedproportionally to how far off-axis it enters the lens 730. Thus, maximumangular separation of the beams, for ATA and ATR, is produced when theoff-axis beam follows the path shown by dashed lines 736. Contiguousangular separation for ECP and ATT is shown by the solid lines 738.

Following the beam separation mirrors 734, the next critical elementencountered by the two beams is a half wave plate 754 which rotates thepolarization (linear) so that they are transmitted by the lag anglecompensator 616. The optical element of the lag angle compensator is anenhanced coated ZnSe plate inclined at the Brewster angle (1.18 rad or67.4°). (The enhanced coating signficantly increases reflectivity to theorthogonal polarization.) This element is the duplexer, referred toearlier, which differentiates between the transmitted and receivedpower.

A 1.5× beam expander 740, after the lag angle compensator, increases thediameter of the beams by 50 percent before they are transmitted by aquarter wave plate 742. The quarter wave plate 742 changes the linearpolarization to circular polarization. The functions performed as thebeams are transmitted through the Scanner/Pointer 620 (and AfocalTelescope) already have been described.

Transmitted laser power reflected from object space, still nominallycircularly polarized, retraces the optical path just described to thelag angle compensator 616. When the reflected power passes through thequarter wave plate 740, it becomes linearly polarized with the plane ofpolarization orthogonal to the polarization of the transmitted input tothe quarter wave plate. Therefore, the received signals are reflected bythe Brewster plate 816 in the lag angle compensator. The lag anglecompensator 616 is rotated by a galvanometer about the long axis shownin FIG. 7 to compensate for the lag angle. The galvanometer is in aclosed-loop servo commanded by a signal derived from a prediction of thelag angle for scanning a flat earth and corrected for deviations fromthis prediction as described earlier in this section. As a result, thesignals reflected by the lag angle compensator 616 follow theessentially fixed path shown in FIG. 7 to the folding mirror 750 (shownabove and to the left of the half wave plate 754).

The above folding mirror 750 reflects the signals to a second foldingmirror 752, directly below the rays shown for the transmitted beams.This second folding mirror 752 is oriented so that in a plan view, therays for the signals appear to be coincident with the rays representingthe transmitted beams. A second beam separation lens (not shown), belowthe mirrors 732 and 734 shown in the figure, performs the inversefunction described for the transmitted beams. The beam separationmirrors extend into the plane of the signals, thereby causing them to bereflected to the two small mirrors below the transmitted beams and atabout 785 mrad (45°) to them. A 95 percent reflectivity beam combiner764 then reflects the signals to the imaging lens 780. The beam combineralso transmits 5 percent of about 1 watt split off from one laser beam,which functions as the local oscillator. The local oscillator signalpasses through lens 766 prior to combining with the signal beams below724 and 732.

The optics for the local oscillator and signals are configured toprovide the required wavefront matching conditions, as described insection 4.1.1. It was mentioned in section 3.0 that the FC Sensoroperates in both the homodyne and heterodyne modes of coherentdetection. Since one-half of the power in each signal originates in thesame laser that provides the local oscillator, homodyne operation isimplemented. However the other half of each signal originates in thelaser that is offset 8 MHz from the laser that provides the localoscillator. Consequently, this combination produces heterodyneoperation.

FIG. 7 includes a simplified functional block diagram of the signalconditioning electronics. The detector preamplifiers, (not shown), areinstalled in the detector/dewar assembly 782; thus, the externalamplifier 784 shown connected to the detector/dewar is the postamplifier. These amplifiers have a bandwidth of 100 MHz to accommodatethe doppler frequency offset due to the forward speed of the hostaircraft. Remembering that the signal imaged on each detector originatesfrom one-half of the output of each laser, one signal at the output ofthe detector will be the doppler offset due to aircraft motion (assumingthe beam is not scanning a moving target. This signal is the differencefrequency between the laser used for the local oscillator and itsreflected counterpart. The second signal at the output of the detectoris the doppler offset due to aircraft motion, plus 8 MHz; i.e., thesecond laser is offset 8 MHz above the reference laser.

A local oscillator signal, equal to 19 MHz+(2V/λ)×10⁻⁶ (V-aircraftradial velocity along the beam axis: λ-wavelength of the laser) at themixer following the postamplifier 784, heterodynes the detector outputsignals in unit 786 to a 19 MHz I.F. bandpass filter 790 (BPF No. 2) andan 11 MHz I.F. bandpass filter 788 (BPF No 1). The bandwidth of theseintermediate frequency video channels is 7.5 MHz to accommodate radialtarget velocities up to ±70 Km/Hr. Frequency shifts due to target motion(i.e., MTI) are detected by means of 28 bandpass filters and associatedelectronics depicted by the row of functional blocks following BPFNo. 1. Each of the bandpass filters 788 and 790, with a bandwidth of 500kHz, is centered at a frequency that results in 250 kHz overlap ofadjacent filters. In this way, target velocity can be resolved toslightly less than 5 Km/hr.

The two IF signals are mixed in unit 792 (i.e., heterodyned) as shown toobtain the 8 MHz difference frequency. After filtering in filter 794 toeliminate the unwanted products from the mixer, the 8 MHz signalprovides one input to a digital phase detector 796. Because this 8 MHzsignal is coherently related to the 8 MHz reference signal in thewavelength offset servo, as described above, the difference in phasebetween them is a measure of relative range. Either the signal from the8 MHz crystal controlled reference oscillator in the wavelength offsetservo or the detected frequency difference between the two lasers isavailable for use as the second (reference) input to the phase detector.That is, over an ambiguity interval of 18.75 meters, the output of thephase detector 796 is proportional to the location of an object beingscanned within that interval.

The output of the bandpass filter 788 is also processed through an MTIfilter 802, an envelope detector 804, a set of low-pass filters 806, anda target logic unit 808, to provide a target detect signal on line 810.

Only the set of video electronics for one beam, is shown in FIG. 7. Asecond set, which duplicates those shown, is provided for the signalreceived for the second beam.

There are two more 11 MHz IF channels not shown in FIG. 7. One detectorelement 772, for the off-axis beam, provides the input to one of thesechannels and the other element 774 provides the input to the otherchannel. (The output from both elements for each beam are summed toobtain the input to the IF channels already described.) Both channels,essentially identical, produce a detected signal whose amplitude isproportional to the signal power on the detector for that channel. Thedifference in amplitude between these output signals is divided by theirsum to provide an error signal, independent of received signal power,proportional to the location of the signal on the two detector elements.When the signal is not centered on the pair of detectors, it signifiesthat the lag angle compensation is deficient. The resulting error signalis applied to the servo for the lag angle compensator, with the polarityneeded to center the image on the detector pair.

It was mentioned earlier that the sensor is operated under the controlof a microprocessor and that a second microprocessor is provided toperform the arithmetic intensive operations. These devices operateasynchronously with communication between them under the control of abus arbitrator. The bus arbitrator, with 4K bytes of dedicated RAM, isconfigured to operate like a dual port RAM but without the cost andcomplication of that hardware. A novel approach to program storage, forboth microprocessors, has been incorporated. That is, the program isstored in EPROMs and downloaded to RAM as part of the start-up sequence,instead of using forms of external program storage. (EPROMs are too slowfor program execution.) Another unique feature of the digitalelectronics design, closely associated with the microprocessors, isdigital generation of the waveforms for the raster(s) required by eachmode of operation. The waveforms, stored in EPROMs, are clocked out ofmemory and converted to analog signals at a rate that exceeds thebandwidth of the servo it drives, thereby producing precision controlled(amplitude and frequency) waveforms.

It is understood that certain modifications to the invention asdescribed may be made, as might occur to one with skill in the field ofthe invention, within the scope of the appended claims. Therefore, allembodiments contemplated hereunder which achieve the objects of thepresent invention have not been shown in complete detail. Otherembodiments may be developed without departing from the scope of theappended claims.

What is claimed is:
 1. A dual-beam amplitude-modulated lasertransmitter/receiver suitable for laser-radar applications that isscalable to high powers, comprising:a laser source with two separateindependent laser optical cavities, each operating at its own frequency,a combining beam splitter at which signals from the two cavities aresuperimposed to form two transmitter output beams with each combinedbeam intensity modulated at the laser difference frequency, so that theoutput consists of two beams separated in elevation and with equal beampowers from each laser cavity with intensity modulation of 100%, Opticalmeans following the combining beam splitter for processing andtransmitting the two beams, reflected energy from the two beams beingreceived at the optical means and directed to receiving means, in whicheach beam has its own homodyne detector and separate local oscillator,whereby means associated with each beam path forms a distinct homodynetransceiver.
 2. A dual-beam amplitude-modulated lasertransmitter/receiver according to claim 1, having means for thefrequency of one of the laser sources to be changed in time, so that amodulated output suitable for absolute range measurements and finedoppler is generated.
 3. A dual-beam amplitude-modulated lasertransmitter/receiver according to claim 1, having means for laserinjection locking in conjunction with means for laser frequency trackingto provide laser tracking stability.
 4. A dual-beam amplitude-modulatedlaser transmitter/receiver according to claim 1, in which said opticalmeans includes azimuth scanning means, elevation scanning means, azimuthpointer means, and an afocal telescope.
 5. A dual-beamamplitude-modulated laser transmitter/receiver according to claim 4, inwhich said optical means further includes channel combining prism meansincluding a λ/4 plate for each beam.
 6. A dual-beam amplitude-modulatedlaser transmitter/receiver according to claim 4, in which said opticalmeans further includes dynamic beam combining means including dynamicgalvonometer control means.